In the switching power-supply device of the primary detection type, it is impossible to detect the output voltage by the voltage of the primary-side auxiliary coil when a switching operation is not performed. This means that it is possible to detect the variation of the output voltage only at timing when a switching operation is intermittently performed, in an intermittent oscillation mode for the purpose of low-standby power at no load or light load state. Accordingly, it is impossible to rapidly respond to sudden load change from no load or light load state during the off period of the switching operation. This is a major drawback as a power-supply.
For this reason, a background switching power-supply device disclose a circuit, which detects the reduction of the output voltage due to the sudden load change from no load or light load state and outputs a voltage reduction signal informing the reduction of the output voltage to a secondary coil, and then a switching operation is started on the basis of the voltage reduction signal transmitted to an auxiliary coil from the secondary coil (for example, U.S. Pat. No. 8,125,799 B2 and US 20120134182 A1).